Quality Assessment of Composite Materials using Ultrasonic Non-Destructive Testing Methods

Abstract

Non-destructive ultrasonic evaluation (NDE) is commonly used for assessment of civil infrastructure and characterization of construction materials. It is an efficient technique that could save millions of dollars with respect to traditional intrusive tests. However, limitations regarding the use of NDE techniques are still present. The conventional non-destructive testing (NDT) methods (impact echo, ultrasonic pulse velocity [UPV]) are focused on velocity; therefore, neither the frequency content of the response nor the frequency characteristics of the transmitter signal to a tested material is usually utilized. However, it has been shown that this can lead to misinterpretation of ultrasonic data. Even for the fairly simple method like UPV (where the method is based on the concept of measuring the time of flight for the first arriving ultrasonic wave from one side of the specimen to another), it has been shown that the UPV results may be affected by many factors, such as water-cement ratio, aggregate size, or distribution of moisture. Additionally, traditional wave velocity-based methods are not sufficient for early damage detection (which is an active research field in the non-destructive testing of civil infrastructure) as they use only one data point of information, neglecting the frequency content of ultrasonic signals. Finally, the long-term durability of glass-FRP (GFRP) in concrete remains an unresolved issue. The necessity of reliable NDE techniques for GFRP bars is even more important for in-situ testing of concrete members with GFRP reinforcement because the bars embedded in concrete show no visual deterioration and cannot be cut out of a structure to test in a traditional way. The main objective of this research is to enhance the understanding of the frequency effects on ultrasonic measurements and establish a comprehensive methodology for early damage detection of composite materials (based on wave velocity, attenuation, and dispersion). This research consists of four studies. First, a characterization procedure is developed, using a state-of-the-art laser Doppler vibrometer, to understand the frequency content transmitted by ultrasonic transducers typically used in civil engineering applications. Second, a group of concrete specimens of different diameter and length is tested with a traditional ultrasonic pulse velocity method (using ultrasonic transducers with different resonant frequencies and the laser vibrometer) to evaluate how the frequency content of the recorded ultrasonic measurements changes with different resonant frequency transducers and how it depends on specimen dimensions.vi Third, a new methodology, based on wavelet synchrosqueezed transform (WSST) and both velocity and attenuation approaches, is developed to address an issue of early damage detection in cementitious materials (i.e. concrete elements and cemented sand specimen). The proposed framework is verified with synthetic signals and two real, lab-scale applications. Finally, the functionality of the newly developed ultrasonic procedure (i.e. based on characterized ultrasonic transducers, the WSST, and velocity and attenuation approach) is investigated on progressive damage of glass fibre reinforced polymer specimens. The ultrasonic evaluation is verified with the traditional destructive test (i.e. shear test) and numerical simulations. The characterization procedure, developed for ultrasonic transducers typically used in civil engineering applications, reveals that frequency content, transmitted by the transducers to the tested medium, consists of more than just transducer resonant frequency. The importance of using well-characterized ultrasonic transducers (i.e. including the full frequency content in the NDT evaluation) is demonstrated on the ultrasonic evaluation of concrete elements, cemented sand specimen, and GFRP reinforcing bars. The study of frequency effects is continued with concrete cylinders of different dimensions. Therefore, practical recommendations regarding the minimum specimen length, effects of increasing length and diameter, and limitations regarding the use of high frequencies in the ultrasonic evaluation of concrete elements are given. Next, a framework based on wave velocity and attenuation (including a demonstration of the advantages of applying the wavelet synchrosqueezed transform [WSST]) is proposed for the evaluation of distributed damage (i.e. early damage induced by freeze and thaw cycles in concrete elements) and localized damage (i.e. cemented sand specimen with a subsurface void). The results indicate that the WSST technique has the potential to improve both the detection of distributed damage by up to 52% and localized damage detection by up to 36%. Finally, a progressive deterioration of GFRP reinforcing bars is studied using the developed ultrasonic procedure. The comparison of ultrasonic evaluation based on wave amplitude, destructive shear test, and numerical simulations shows that ultrasonic techniques can successfully predict the degradation of shear strength (and ultimately tensile strength) of GFRP bars (with the maximum error of 7%). The findings presented in this thesis provide practical recommendations and frameworks that can successfully increase the reliability of non-destructive ultrasonic evaluation of composite materials used in civil engineering app